1. Field of the Invention
The present invention relates to a field effect transistor (FET) operated in a microwave band and, more particularly, to a dual gate microwave FET.
2. Related Background Art
A conventional known FET of this type has two gate electrodes aligned in a direction of gate length and is formed on a GaAs substrate (IEEE Transaction on Electron Devices, Vol. ED-22, No. 10 (1975), p. 897).
Only a DC wave or a low-frequency wave equivalent to the DC wave is applied to the second gate in a conventional dual gate microwave FET.
If a microwave can be applied to the second gate, a target conversion signal (f.sub.RF) is applied to the first gate, and a local oscillation signal (f.sub.OSC) is applied to the second gate to obtain a desired signal (f.sub.IN) from the drain, thereby realizing a frequency converter (mixer), as shown in FIG. 1. To the contrary, as a method of realizing a single gate FET having the same function as the above frequency converter, a target conversion signal and a local oscillation signal are applied to the gate electrode. According to this method, however, a separator for separating these two signals from each other is required. In a frequency range exceeding microwaves, this separator is constituted by a transmission circuit. For this reason, a circuit scale is undesirably increased to make it difficult to provide a monolithic IC (integrated circuit). As another method of realizing a frequency converter by using a single gate FET, a target conversion signal is applied to the gate electrode, and a local oscillation signal is applied to the drain. According to this method, however, a circuit for separating the local oscillation signal from an output signal is required.
In order to realize a frequency converter by using a single gate FET, the circuit for separating the target conversion signal from the local oscillation signal or the local oscillation signal from the output signal is required.
To the contrary, if a dual gate FET can be used to arrange such a frequency converter, the separator is not required. In a conventional dual gate FET, however, the microwave cannot be applied to the second gate, as described above. In addition, a noise level of the dual gate FET is higher than that of the single gate FET.